Compressors



Nov. 22, 1966 M. E. CLARKE 3,286,911

COMPRESSORS Filed Sept. 1, 1965 2 Sheets-Sheet l INVENTOR M Kim EL E bu mm Omen:

WMMVW A TTORNE Y5 Nov. 22, 1966 M. CLARKE 3,

COMPRESSORS Filed Sept. 1, 1965 2 Sheets-Sheet 2 l/WENTOR HIGH/YEA Ebwmb Clmkll A TTORNE YS' United States Patent 3,286,911 COMPRESSORS Michael Edward Clarke, Kingston, England, assignor to The British Oxygen Company Limited, London, Eng- Filed Sept. 1, 1965, Ser. No. 484,410 Claims priority, application Great Britain, Sept. 4, 1964, 36,451/64 14 Claims. (Cl. 230-55) This invention relates to compressors and more specifically to reciprocating piston compressors driven by linear electric motors.

Compressors are known comprising a cylinder, a piston in the cylinder, a coil spring co-axial with the cylinder and attached at one end to the piston and anchored at the other end with respect to to the cylinder, a coil co-axial with and attached to the piston and means for producing a magnetic field co-ax-ial with and directed generally radially of the axis of the cylinder and directed through the coil. Upon alternating current being passed through the coil, the radially directed magnetic field acts on the alternating current flooring in the coil and causes the unitary coil and piston to reciprocate.

Such known compressors have the disadvantage that they are not dynamically balanced and either vibrate relative to their surroundings, if resiliently mounted, or transfer vibrations to their surroundings, if fixedly mounted. Such vibrations are known to result in damage to such fittings external to the cornpressor, as conduits.

It is an object of the present invention to overcome the aforementioned and other disadvantages in known forms of compressors driven by linear electric motors.

According to the present invention there is provided a compressor comprising two cylinders rigidly connected together and located in axial alignment, a piston in each cylinder, a spring connected at its ends to the pistons or two springs each connected at one end to a respective one of the pistons and anchored at the other end with respect to the cylinders, means for producing two magnetic fields substantially radial of the axis of the cylinders and through a portion of each piston or an axial extension thereof, two coils located one in each of the radial fields and co-axial with and rigidly connected one to each of the pistons, the means for producing two magnetic fields substantially radially of the axis of the cylinders co-operating with the coils when an alternating current is supplied either to the means for producing the radial magnetic fields or to the coils or to both, to form two linear motors serving to reciprocate the pistons in antiphas'e relationship, and valve means for allowing flow of the gas to be compressed into each cylinder and flow of compressed gas trom each cylinder.

Conveniently the cylinders are arranged in axially aligned opposition.

The gas compressed in one of the cylinders may be led to the other cylinder for further compression and in this case the two cylinders may be of differing cross-sectional areas.

Preferably it is arranged that in operation the alternating current supplied to the coils, when the magnetic fields substantially radial of the axis of the cylinders are nonalternating fields, has a frequency substantially equal to the natural frequency of the piston and spring assembly.

When alternating currents are supplied to the coils and 3,286,91 l Patented Nov. 22, 1966 to the means for-producing the two magnetic fields and when these two alternating currents have the same frequency it is arranged that the two currents are inphase and this may be achieved by connecting the two coils in series.

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIGURE 1 shows in axial section a two-stage compressor in accordance with the present invention.

FIGURE 2 shows diagrammatically an axial section of a further two-stage compressor in accordance with the present invention.

The two stage compressor shown in FIGURE 1 comprises a tubular housing 1 to the ends of which are attached by bolts 2, plates 3 of annular shape and formed of magnetic material. The plates 3 have apertu res 4 coaxial with the tubular housing 1. Cylinders 5 and 6 are attached to the remote faces of the plates 3 and are located coaxially with the apertures 4 in the plates 3 and hence with one another. The cylinders 5 and 6 are closed at their remote ends by cylinder heads 7 and 8. Each cylinder head 7 and 8 has formed therein an inlet port 9 and an exhaust. port 10 and includes pressure-operated inlet and outlet valves 11 and 12 respectively, all springbiased towards a closed position. The cylinders 5 and 6 and the cylinder heads 9 and 10 are provided with passages 13 for flow of coolant.

The working space within the cylinder 5 has a greater cross-sectional area than the working space within the cylinder 6 since the cylinder 5 constitutes the cylinder of the low pressure stage and the cylinder 6 constitutes the cylinder of the high pressure stage of the two-stage compressor. 7

A conduit 14 extends from the exhaust port 10 of the low pressure stage to a heat exchanger 15 and a conduit 16 extends .from the heat exchanger 15. to the inlet port 9 of the high pressure stage. Conduits 17 and 18 communicate with, for example, a gas cleaner and drier and with a pressure storage vessel, or the like, respectively;

Disposed in the cylinders 5 and 6 arepistons 19 and 20 respectively, each having two piston rings. pistons 19 and 20 are of different diameters each appro priate to their related cylinders 5 and 6. Each piston 19 and 20 has a tubular skirt portion 21 of a diameter greater than the diameter of the pistons. The tubular skirt portions 21 extend through the apertures 4 inthe annular plates 3 and have formed on their free ends radially directed flanges 22.

Located within the housing 1 and at the middle of the axial length thereof is a barrier plate 23 secured to the housing 1. Secured one to each side of the barrier plate 23 are a pair of pole-pieces 24. The pole-pieces 24 have the :form of a mandrel extending in an axial direction and are each of a length to extendfrom'the barrier plate 23 through the aperture 4 in the relatedend plate 3 so as to'terminate at the side of the end plate 3 remote from the barrier plate 23. A portion of each pole-piece 24 is encompassed by the skirt 21 of the related piston 19 or 20 and at least this portion of each pole piece is of a diameter less than the internal diameter of the skirt 21. The pole-pieces'24 have formed at their ends adjacent the barrier plate 23 radially directed flanges 25. Mounted between the flanges 25 of the pole pieces 24 and the flanges 22 on the piston skirts 21 are The two coil springs 26 coaxial with the pole-pieces 24. The springs 26 are secured at one end by 'bolts 27 "to the barrier plate 23 and at the other end to the piston skirts 21. The springs 26 are, in this case, formed by machining a helical slot in a tube.

Located :within the annular spaces between the springs 26 and the housing 1 are two electromagnets 28 of annular form in section and coaxial with the axes of the cylinders and 6. The electromagnets 28 are secured to the end plates 3 by bolts 29. i Located in recesses 30 formed in the radially outer surfaces of the piston skirts 21 are two coils 31 and 32. Electrical leads 33 for the coils 31 and 32 extend from the exterior of the apparatus through an aperture 34 in the housing 1, bores 35, around the springs 26 to the coils 31 and 32.

In operation, direct current is supplied to the electromagnets 28 and a magnetic field is thereby formed between the end plates 3 and the pole-pieces 24. These two magnetic fields are directed general-1y radially of the polepieces 24, that is, normal to the axes of the cylinders 5 and 6. Alternating current is supplied through the leads 33 to the coils 31 and 32.

It is arranged that at any instant the effects of the radially directed magnetic fields on the currentflowing in the coils 31 and 32 is such that the coils 31 and 32 and hence the pistons 19 and 20 tend to move in opposite directions. This is arranged either by ensuring that the instantaneous directions of flow of the current in the coils 31 and 32 are in the same sense about the common axis of the coils and that the directions of the magnetic flux of the radially directed fields are in opposite senses, that is the flux is directed radially inwards in one of the radially directed fields, and is directed radially outwards in the other radially directed field, or the magnetic fluxes of the two radially directed fields are directed in the same sense and the instantaneous flows of current in the coils 31 and 32 are in opposite senses about the common axis of the coils 31 and 32.

Since alternating current is supplied to the coils 31 and 32, the axially directed forces acting on the coils and hence the pistons also alternate in direction and the pistons reciprocate in their cylinders.

Upon the pistons 19 and 20 approaching one another the pressure in each cylinder decreases and the inlet valves 11 open due to the pressure differential on the two faces of each valve 11 overcoming the spring-bias of the valve 11 to the closed position. Gas to be compressed in the first stage flows through the conduit 17 and port 9 to the space within the cylinder 5 and gas in the conduits 14 and 16 and the heat exchanger 15 having been compressed in the first stage flows through the port 9 into the space within the cylinder 6. Uponthe alternating current reversing its direction the pistons 19 and move away from one another and compress the gas in their respective cylinders. As the pressure of the gas in the space in the cylinder 5 becomes greater than the pressure of gas in the port 10, conduits 14 and 16 and heat exchanger 15 the valve 12 in the cylinder head 7 opens and allows gas to flow from the cylinder 5. Likewise, as the pressure of gas in the space in the cylinder 6 becomes greater than the pressure of gas in the conduit 18 and its associated port 10 the valve 12 in the cylinder head 8 opens and allows gas now having been compressed in both stages to flow from the cylinder through the port 10 and conduit 15. Upon reversal of direction of movement of the pistons 19 and 20 the valves 12 close and subsequently the valves 11 open.

In the absence of a resonant system the ratio of the work done in reciprocating the pistons to the work done in compressing the gas would be undesirably high, that is, the efliciency of the machine would be very low. However, by making the natural frequency of the piston, and spring system correspond to substantially the .frequency of the alternating current to be applied to the coils, the

effective mass of the pistons, in the absence of any damping of movement of the pistons, is reduced substantially.

to zero and the work done by the alternating current is then primarily done in compressing gas. Thus by constructing and operating the machine so that the natural frequency of the spring and piston system corresponds substantially to the frequency of the alternating current supplied to the coils, a machine of high efficiency is provided.

In order that the machine or its surroundings shall. not be vibrated by operation of the machine it is. necessary that all moving parts to the left of the barrier plate 25. be 1 dynamically balanced with the moving partsto the right of the barrier plate 25 as seen in FIGUREI. It. is also necessary that the resonant frequencies of the spring 26 i and piston 19 system and of the spring 26 and piston 20 L This system remain substantially constant and equal. may be achieved by making the pistons 19 and 20 heavy so ensuring that changes in the forces applied to the springs 26 by gas being compressed represent only a small fraction of the total forces applied to the springs by the gas and by the kinetic energy of the pistons;

The compression and delivery of gas has a damping effect upon the reciprocating pistons but provided the i drive power of the linear motors constituted in part by the electromagnets 28 and the coils 31 and 32 is ade quate, such damping has little effect on the resonant frequencies of the piston and spring systems.

The constancy of the resonant frequency could be impaired if the dead volume of the cylinders, that is, the l volume of the gas space within the cylinders when the pistons are at the end of their compression strokes, were to be too large. Gas remaining in the dead volumeis at high pressure and exerts a force .on the related piston which is in the same direction as the force exerted on the. piston by the related spring and hence the gas in thecylinder performs work on the piston and tends to raise the resonant frequency of the piston and spring systems.

However, the tendency of gas present in the dead volume.

of each cylinder at the end of a compression stroke. to

siderably exceed forces due to compression of gas,

the peak force exerted by gas on the pistons.

In the embodiment shown in and described withreference in FIGURE 1 the springs 26 are formed by machining a helical slot in a tube. However, rod or wire springs.

maybe used and in order to reduce the inertia, tubular springs may be used.

FIGURE 2 shows diagrammatically a further embodiment of the present invention. when the arrangement shown in FIGURE 1 is in operation with the two piston and spring systems operating at 1 the same resonant frequency there is no resultant force 1 acting on the barrier plate 23 either to the left or to. the Thus it would be possible i to omit the barrier plate 25.an'd to substitute a single. spring for the two springs 26 there then being a .nodal plane at the centre of the axial length of the housing 1, that is, in a plane otherwise enveloped by the barrier plate 23. Such an arrangement is shown in FIGURE 2..

In FIGURE 2 there is shown an electromagnet40 of right as seen in FIGURE 1.

tubular form and having pole-pieces 4 1. To the ends of the electromagnet. 40 are connected cylinders 42 and 43 1 having valves 44 in their haads. The spaces within the 1 cylinders 42 and 43 are, in this embodiment also, of differing cross-sectional areas.

pole piece 49 supported from the electromagnet 40 by It will be realised that:

Located in the cylinders 1 42 and 43 are pistons 45 and 46 having attached thereto coils 47 similar to the coils 31 and 32 described with reference to FIGURE 1. A single coil spring 48 extends be: tween and is connected to the free ends of the coils 47. Located radially within the coils 47 and coil spring .48 is a means of supports (not shown) extending through the coil spring 48 at the middle of the axial length of the spring 43, that is, the supports are located in the nodal plane of the spring 48.

The operation of the arrangement shown in FIGURE 2 is similar to that of the arrangement shown in FIGURE 1. The electromagnet 40 is caused "to create magnetic fields across the annular gaps between the pole-pieces. 41 and the pole-piece 49. Alternating current is passed through the coils 47 so that the pistons 45 and 46 tend to move in anti-phase relationship.

Whilst in the above described embodiments the mag-- netic fields in which the coils 31 and 32 or 47 are located are described as being created by electromagnets 28 or 40 it is to be understood that permanent magnets may be used. However, the use of electromagnets excited by direct current enables the use of comparatively compact and low weight components for producing the magnetic field through the coils 31 and 32 or 47. Electromagnets excited by alternating current may be used in place of the electromagnets 28 or 40 excited by direct current. In this case it is arranged that alternating currents in the coils of the electromagnets and in the moving coils are in phase. In the case where alternating currents of the same frequency are supplied to the electromagnets and to the coils it is necessary to arrange that the natural frequency of the piston and spring system is twice the frequency of the alternating currents.

Whilst the etficiency of a compressor according to the present invention may be lower than that of a conventional induction motor it is high when compared to the eficiency of a compressor unit comprising a rotary motor and one or more reciprocating pistons.

It is possible that with some conditions of operation of the embodiments described that there may be a small amount of unbalance between the high-pressure and lowpressure ends of the machines and this leads to the pistons reciprocating slightly out of antiphase relationship. If this occurs the balance may be improved or restored by variation of the current in one or both of the coils.

Compressors constructed according to the present invention are well suited for operation at ambient temperatures and lower, and the absence of bearings, shaft seals, connecting rods and of liquid lubricants are important advantages in thatthey enable contamination of the gas to be avoided.

What is claimed is:

1. A compressor comprising:

a first cylinder;

a second cylinder rigidly connected to and located in alignment with said first cylinder;

two pistons located one in each of said cylinders;

spring means connected to the pistons;

means producing two magnetic fields substantially radial of the axis of the cylinders and through a portion of each piston;

two coils located one in each of the radial magnetic fields and coaxial with and rigidly connected one to each of said pistons and co-operating with said means producing two magnetic fields substantially radially of the axis of the cylinders when electric current is applied to the coils to form two linear motors reciprocating the pistons in antiphase relationship;

and valve means allowing flow of the gas to be compressed into each cylinder and flow of compressed gas from each cylinder.

2. A compressor comprising:

a first cylinder;

a second cylinder rigidly connected to and located in axial alignment with said first cylinder;

two pistons located one in each of said cylinders;

two springs each connected at one end to a respective one of said pistons and anchored at the other end with respect to said cylinders;

means producing two magnetic fields substantially 6 radial of the axis of the cylinders and through a portion of each piston;

two coils located one in each of the radial magnetic fields and coaxial with and rigidly connected one to each of said pistons and cooperating with said means producing twomagnetic fields substantially radially of the of thecylinders when electric current is appliedto the coils to "fon'n'twoline'ar motors reciprocating the pistons in antiphase relationship;

and valve means allowing flow of the gas to be compressed into each cylinder and flow of compressed gas from each cylinder.

3. ,A compressor as in claim 2, wherein:

the said cylinders are in axially aligned opposition.

4. A compressor as in claim 2, including:

a conduit extending between the exhaust port of one of said cylinders and the inlet port of the other of said cylinders.

5. A compressor as in claim 2, including:

a conduit extending between the exhaust port of one of said cylinders and the inlet port of the other of said cylinders;

and wherein the said one cylinder has a larger crosssectional area than the said other cylinder.

6. A compressor as in claim 2, including:

a conduit extending between the exhaust port of one of said cylinders and the inlet port of the other of said cylinders;

and a heat exchanger in the said conduit between the exhaust port of the said one cylinder and the inlet port of the said other cylinders 7. A compressor as in claim 2, wherein: the said means for producing two magnetic fields substantially radial of the axis ofthe cylinders comprises:

two electromagnets each having: a. first pole piece of annular form surrounding a respective one of the said two coils; and a second, central pole piece extending within the same respective one coil. 8. A compressor as in claim 2, wherein: each of said springs is a helically cut tube. 9. A compressor as in claim 2, including:

means for varying the current supplied to one of the said coils.

10. A compressor comprising:

a first cylinder;

a second cylinder rigidly connected to and located in opposition to and in axial alignment with said first cylinder;

two pistons located one in each of said cylinders;

a spring connected at its end to the two pistons;

means producing two magnetic fields substantially radial of the axis of the cylinders and through a portion of each piston;

two coils located one in each of the radial magnetic fields and coaxial with and rigidly connected one to each of said pistons and co-operating with the said means producing two magnetic fields substantially radially of the axis of the cylinders when an electric current is supplied to the coils to form two linear motors reciprocating the pistons in antiphase relationship; and

valve means allowing flow of the gas to be compressed into each cylinder and fiow of compressed gas from each cylinder.

11. A compressor as in claim 10, wherein:

one of said cylinders has a larger cross-sectional area than the other of said cylinders.

12. A compressor as in claim 10, wherein:

the said means for producing two magnetic fields substantially radial of the axis of the cylinders comprises:

two electromagnets each having:

a first pole piece of annularform surrounding a References Cited by the Examiner a iiififi llfifiififiii iiiii ixiil fiifigiitmn the UNITED STATES PATENTS same -g' One ml 872,101 2/1959 Ryba 23055 13. A compressor as in claim 10, wherein: 5 ORE GN PA the said spring is a helically cut tube. 978,600 4/1951 Franca 14. A compressor as in claim 10, including: means for varying the current supplied to one of said ROBERT WALKER Examinen coils. e 

1. A COMPRESSOR: A FIRST CYLINDER; A SECOND CYLINDER REGIDLY CONNECTED TO AND LOCATED IN ALIGNMENT WITH SAID FIRST CYLINDER; TWO PISTONS LOCATED ONE IN EACH OF SAID CYLINDERS; SPRING MEANS CONNECTED TO THE PISTONS; MEANS PRODUCING TWO MAGNETIC FIELDS SUBSTANTIALLY RADIAL OF THE AXIS OF THE CYLINDERS AND THROUGH A PORTION OF EACH PISTON; TWO COILS LOCATED ONE IN EACH OF THE RADIAL MAGNETIC FIELDS AND COAXIAL WITH AND RIGIDLY CONNECTED ONE TO EACH OF SAID PISTONS AND CO-OPERATING WITH SAID MEANS PRODUCING TWO MAGNETIC FIELDS SUBSTANTIALLY RADIALLY OF THE AXIS OF THE CYLINDERS WHEN ELECTRIC CURRENT IS APPLIED TO THE COILS TO FORM TWO LINEAR MPTORS RECIPROCATING THE PISTONS IN ANTIPHASE RELATION; AND VALVE MEANS ALLOWING FLOW OF THE GAS TO BE COMPRESSED INTO EACH CYLINDER AND FLOW OF COMPRESSED GAS FROM EACH CYLINDER. 